Copper leadframes of encapsulated semiconductor packages can corrode during the manufacturing and assembly process. This is undesirable as oxidation and corrosion of the copper surface leads to poor electrical contacts between both the semiconductor chip and the leadframe, as well as, between the leadframe and the external circuit board.
One method of improving the reliability of semiconductor packages is to coat the copper leadframe with a thick silver coating. The silver coating takes up approximately 40% of the cost of the frame, thereby limiting potential future price reductions. More importantly, high quality silver coatings can be electroplated only by using cyanide-based plating solutions. The use of cyanide in manufacturing processes is undesirable because of its high toxicity and alternatives are sought.
U.S. Pat. No. 6,376,901 uses a series of electroplated coatings, first coating the whole leadframe with a Ni coating and in an additional two steps, selectively coating the leadframe with two palladium layers of different thickness. This process is undesirable in that at least three plating steps are required. In addition, palladium is also extremely expensive thus adding to the overall cost of the process.
U.S. Pat. No. 6,545,342 creates a coating over the entire copper leadframe by immersing the leadframe in a chromating solution. A percentage of the copper metal atoms are converted into substitutional metal complexes. The composition of this layer is very complicated and difficult to closely define. In addition, to produce a leadframe with the best properties, a copper layer must be initially deposited.